The following is a list of prior art publications believed to be pertinent:
1. "Proceedings of the new concepts symposium and workshop on detection and identification of explosives, Reston, Virginia", U.S. Department of Commerce, National Technical Information Service (Oct. 1978), and references therein; PA1 2. U.S. Pat. No. 3,832,545, J.R. Hansen, W.C. Divens, M.J. Hurwitz and R.J. Schneeberger, Westinghouse Corporation; PA1 3. "An evaluation of X-ray fluorescence as a means of explosives detection", R.I. Miller, C.L. Abbat, S.A. Daavey and P.T. Smith, SAI, Sunnyvale, Ca 1977. The work was supported by the FAA under contract DOT-FA77WA-3963; PA1 4. F.S. Goulding, Muc. Instr. and Meth. 142 1977) 213; PA1 5. Differential X-ray method and apparatus, F. Keliz and C.A. Mistretta, Wisconsin Alumni Research Foundation, U.S. Pat. No. 3,974,386; PA1 6. Determination of uranium content in material quantitative analysis, R.C. Smith, Westinghouse Electric Corp. U.S. Pat. No. 4,158,769 (1979).
A great need exists for the fast screening of luggage, parcels, letters and the like for the purpose of detecting any concealed explosive devices (Ref.1). Explosive devices comprise an explosive material and a detonator and the former are often in the form of sheets such as those commercially known by the name Deta (Trade Mark, Du Pont) which are difficult to detect by any known fast scanning system (Ref.2). It has therefore been realised that a reliable method for the detection of explosive devices such as booby-traps or time bombs, that functions regardless of the nature and shape of the explosive material, could be based on detection of their lead-based or mercury-based detonators. Due to their small size, detonators are easily concealed among other objects. Consequently they cannot generally be perceived visually, e.g. in images of the interior of a piece of luggage produced by a conventional X-ray apparatus such as used for the inspection of luggage at airports. It has therefore been proposed (Ref. 3) to detect lead-based or mercury-based detonators by means of the X-ray fluorescent technique (Ref.4). This method, however, was found to be ineffective in luggage handling operations because of its low sensitivity, the high probability of its producing false alarm and its being intrinsically incapable of imaging the screened object.
There is known in the art the so-called K-edge technique for the detection of elements by means of X-rays. Thus for example, U.S. Pat. No. 3,974,386 (Ref.5) discloses the improvement of the visibility of a contrast medium, such as iodine or xenon by making use of their K-edge absorption. In accordance with that method the patient with the particular absorption medium under consideration in some of his internal parts is kept still and serves as the substrate that produces a K-edge discontinuity of the transmitted radiation. The X-ray images of the patient resulting from the X-ray transmittance, are directly intercepted by an intensification screen adapted to convert an invisible X-ray image into a visible image. Several different images are produced with different X-ray intensities only one of which corresponds to the K-edge energy and the resulting visible images produced by the intensification screen are processed by a complicated substraction and integration process by which a visible image with the desired contrast intensification is obtained. Thus, essentially this is a conventional X-raying method with an attachment for improving the visibility of contrast media.
Another detection method based on the K-edge technique is disclosed in U.S. Pat. No. 4,158,769 (Ref.6). That method serves for the determination of the uranium content in a solution obtained by extraction of a uranium ore and use is made of the intrinsic .gamma.-radiation of thorium, a uranium daughter that invariably accompanies uranium. The inspected solution is flown past a window that is transparent to .gamma.-radiation and on the other side of that window there are located immobile .gamma.-radiation filtering means and the radiation emerging from such filtering means is evaluated by suitable radiometric means. The filters in association with the radiometric means are designed to respond to absorption at the K-edge energy band which produces a K-edge discontinuity and in this way it is possible to obtain an indication of the uranium content in the inspected solution. The .gamma.-radiation here is intrinsic and originates from within the observed object. Put in other words, the quantity of a soughtafter object is here determined on the basis of an intrinsic property thereof.
There is not known any method or apparatus for the detection of heavy elements or detonators by the K-edge technique in which the presence or absence of a given substrate even in a complete matrix is determined on the basis of a K-edge discontinuity of the transmittance of impinging extrogeneous X-radiation in combination with a detector system of high resolving power.